In modern vehicles equipped with a catalytic converter, the warm-up phase takes place at high intake manifold pressures and at high exhaust-gas temperatures. In order to warm up the catalytic converter as quickly as possible, the engine is driven with a greatly retarded injection angle for a cold start, that is, with poor engine efficiency. The desired heating operation on the catalytic converter results as a consequence of an increased exhaust-gas temperature. The poor engine efficiency is compensated by an increased throughput of the air/fuel mixture which is caused by driving to open the throttle flap or the idle adjusting element. In this way, and compared to normal idle or part-load operation, a significantly increased manifold pressure or a significantly reduced manifold pressure results. Catalytic converter warm-up functions of this kind are known, for example, from U.S. Pat. No. 5,184,463.
The above-described increase of the manifold pressure is further increased if, at the same time, a lean operation of the engine is desired. This leads to the problem that, under certain circumstances, an inadequate manifold pressure is available for the braking-force amplifier. Auxiliary-force braking units can operate with manifold underpressure as an energy store. These auxiliary-force braking units are widely used for passenger cars and for light-weight and medium-weight size commercial vehicles.